This invention relates to an electromagnetic rotation sensor, and more particularly to a construction of fixing a cover of the rotation sensor.
There is known an electromagnetic sensor of a construction described below. As shown in FIG. 3, this sensor comprises a sensor portion 2 having a detection portion 1, and a connector portion 3 having contacts 3a for connection to an external circuit, the contacts 3a being exposed at the upper end of the sensor portion 2. The sensor portion 2 includes an upper larger-diameter portion 2a, and a lower smaller-diameter portion 2b having the detection portion 1 disposed in eccentric relation to the larger-diameter portion 2a. The sensor portion 2 and the connector portion 3 are molded of a resin integrally with each other in such a manner that the connector portion 3 has an socket portion 3b into which a mating connector is adapted to be inserted. A cover 6 includes a smaller-diameter portion 6a for receiving the larger-diameter portion 2a of the sensor portion 2, and a larger-diameter portion 6b for receiving a detection gear connected coaxially to a rotating member whose rotational speed is to be detected or measured, the smaller-diameter portion 2b of the sensor portion 2 being eccentric from the larger-diameter portion 6b of the cover 6. A sensor portion-positioning hole 6c is formed through an upper end wall of the cover 6. As shown in FIGS. 4A and 4B, the larger-diameter portion 2a of the sensor portion 2 is inserted into the smaller-diameter portion 6a of the cover 6 through a packing 2c, with the connector portion 3 extending through the sensor portion-positioning hole 6c. Then, the smaller-diameter portion 6a of the cover 6 is compressively deformed at a plurality of circumferentially-spaced points or over the entire circumference thereof (In FIG. 4, this portion 6a is compressively deformed over the entire circumference thereof, as shown by 7, thereby fixing the cover 6 to the sensor portion 2 to prevent the cover 6 from becoming disengaged from the sensor portion 2. The sensor is used in the following manner to detect the rotational speed (the number of revolution) of the rotating member.
As shown by broken lines in FIG. 4B, a connection portion 6d of the cover 6 is fixed to a rotating member guide 4 (which guides the above rotating member), so that the surface of the detection portion 1 of the sensor portion 2 is disposed in opposed relation to the detection gear 5 fixedly mounted on a rotation shaft 4a coaxial with the rotating member guide 4. For example, the detection gear 5 has alternate radial projections 5a and recesses 5b arranged at equal intervals, as shown in FIGS. 5A and 5B. A change in the gap between the detection portion 1 and the detection gear 5 in response to the rotation of the rotation shaft 4a (that is, a change in magnetic flux) is converted into pulses, and is outputted to the connector portion 3, and the rotational speed is measured, for example, by counting the number of output pulses for a predetermined time period.
In this conventional rotation sensor, in order to obtain accurate measurement results, the centerline (axis) of the detection portion 1 should be parallel to the centerline of the cover 6, and also the larger-diameter portion 6b of the cover 6 having the connection portion 6d should be coaxial with the detection gear 5, so that the gap between the detection portion 1 and the projections 5a of the detection gear 5 can always be kept constant during one revolution. For example, when the gap between the surface of the detection portion 1 and the end surface of the projection 5a of the detection gear 5 varies, the value of the output of the sensor portion becomes small, so that the detection accuracy is worsened. In addition, in the conventional sensor, even if dimensional accuracies of the cover 6 and the sensor portion 2 are enhanced, it is difficult to prevent a positional displacement between the cover 6 and the sensor portion 2 when assembling and fixing them together. Therefore, it is difficult to satisfy the above requirements, and also since the positional displacement can vary from one product to another, variations in characteristics from one product to another can not be avoided.
More specifically, in the conventional electromagnetic rotation sensor, the cover 6, when compressively deformed for clamping purposes, is deformed such that the longitudinal dimension thereof is changed, and also the sensor portion 2 is displaced out of position. And besides, the longitudinal dimension of the cover 6 is varied depending on variations in the compressive deformation force. Further, the cover 6 may be distorted.
For these reasons, it is difficult to control the gap between the detection gear 5 and the detection portion 1 of the sensor portion 2 to a constant value when assembling the sensor, and therefore the electromagnetic rotation sensor of a high precision can not be obtained. The problem caused by the positional displacement between the cover 6 and the sensor portion 2 due to the compressive deformation is also encountered with the type of sensor (FIG. 6) in which a sensor portion 2 is molded into a right cylindrical shape, and a cover 6 has a right cylindrical shape.